1. Field of the Invention
The present invention relates to an apparatus for detecting a malfunction in an air-fuel ratio control operation for an internal combustion engine.
2. Description of Related Arts
In a known system for an internal combustion engine for controlling an air-fuel ratio in accordance with a detected air-fuel ratio by an air-fuel ratio sensor, an appropriate operation of the air-fuel ratio sometimes cannot be obtained due to a change in a characteristic after use or a tolerance, or to a malfunction. Therefore, it was proposed, in "Toyota Gijyutsu Kokai Syu (Collection of Published Technology of TOYOTA), volume number 1667, published on Jan. 29, 1988, to use a system wherein a product of a value of an air-fuel ratio correction factor, calculated as a value corresponding to an air fuel ratio in accordance with detected values of air-fuel ratio sensor, and a value of a learning correction factor, is calculated, and it is determined that a malfunction of the air-fuel ratio control has occurred when the value of the product of the air-fuel ratio correction factor and the air-fuel ratio learning correction factor is high.
This prior art suffers from a drawback in that a malfunction of the air-fuel ratio control cannot be detected when a plurality of causes of malfunctions occur simultaneously. Namely, the above system allows a detection of a malfunction caused when a single factor, such as a change in a characteristic of an air flow meter, occurs. But when, in addition to this malfunction related to the air flow meter, another malfunction, such as a change in the flow characteristic of a fuel injector after prolonged use or a changed tolerance of parts is added, the effects of these combined malfunctions sometimes negate each other, so that a malfunction of the air-fuel ratio control cannot be detected regardless of the fact that the air-fuel ratio control is not correct.
Furthermore, if a malfunction of an injector of one of the cylinders occurs, the exhaust gas from this cylinder is usually properly mixed with the exhaust gas from the remaining cylinders before it reaches the air-fuel ratio sensor. This means that the exhaust gas from the faulty cylinder can have only a very small effect on the air-fuel ratio correction factor to be calculated from the air-fuel ratio signal output by the air-fuel ratio sensor, since the exhaust gas from the faulty cylinder is "diluted" by the exhaust gas from the other unaffected cylinders, and thus a detection of a malfunction of the air-fuel control cannot be property effected.
This problem of a difficulty in detecting a malfunction grows larger when the air-fuel ratio control system has two air-fuel ratio sensors; one arranged upstream of a catalytic converter and the other arranged downstream of the catalytic converter. This double air-fuel ratio sensor system has a sub-feedback system for controlling the feedback correction factor in accordance with a signal from the second sensor, so that the amplitude of the feedback correction factor is decreased. Therefore, an impermissible amplitude of the feedback correction factor generated by a malfunction is apt to be suppressed by the operating of the sub-feedback system, and thus a quick and precise detection of a malfunction in one cylinder of the engine becomes difficult.